a) Field of the Invention
The present invention relates to a method for controlling an injection unit of an injection molding machine so as to knead and plasticize a resin material into a uniform molten resin ready for injection into an associated mold.
b) Description of the Related Art
In an in-line screw type injection unit, as is known well in the art, rotation of a screw in one direction within a heating screw causes a resin material, which has been charged into the heating cylinder from a hopper, to advance along threads of the screw to a front end portion of the heating cylinder while being kneaded.
The resin material is heated, plasticized and molten by heat conducted from the heating cylinder which is heated by a band heater and also by frictional heat developed within the resin material and between the resin material and associated metal surfaces owing to the kneading action of the screw.
As long as plasticization of a resin material is preformed depending solely on the above-described resin plasticization mechanism which relies upon external heating and simple unidirectional rotation of a screw, it is difficult to uniformly plasticize all kinds of resin materials, which vary widely, no matter how conditions for plasticization such as screw revolution speed, external heating temperature and back pressure are modified. To meet various requirements inherent to such resin materials, there has heretofore been no choice other than achieving the uniform plasticization by changing the screw design, for example, L/D (effective screw length/screw diameter), P/D (pitch between screw threads/screw diameter), the proportions of feed zone, compression zone and metering zone of a screw, the depth of threads in each zone of the screw, etc.
However, to retain many screws of different designs as described above for an injection unit of an injection molding machine of the above-described sort, which is desired to achieve kneading and plasticization of a wide range of resin materials by commonly using a single screw, is not advantageous from the standpoints of economy, cumbersome replacement and the like. Moreover, there is a limitation to the achievement of more uniform plasticization as long as modifications in the screw design are exclusively relied upon.